Soybean crop outran disease: Rust does little damage in Southeast

Drought and Asian soybean rust don't get along too well. Unfortunately, neither do drought and profitability, but at least soybean growers had few problems with rust in the 2007 season.

In the winter of 2006/07, soybean rust over-wintered farther north than in the two previous years the disease was closely monitored. The presence of rust farther north led some to believe 2007 would be a bad rust year. However, extreme drought and heat blunted development and movement of the disease in the Southeast.

While most of the tropical systems that typically hit the Southeast hit farther west, late season weather fronts finally pushed rust northward from Florida and Georgia.

By November 2007, rust had been detected in 19 states and 267 counties including: 23 counties in Alabama (19 soybean), 33 counties in Arkansas (soybean), 21 counties in Florida (11 soybean), 21 counties in Georgia (14 soybean), four counties in Illinois (soybean), one county in Indiana (soybean), 14 counties in Iowa (soybean), nine counties in Kansas (soybean), three counties in Kentucky (soybean), 20 parishes in Louisiana (18 soybean), 23 counties in Mississippi (21 soybean), 35 counties in Missouri (soybean); four counties in Nebraska (soybean); 12 counties in Oklahoma (soybean), seven counties in South Carolina (soybean), six counties in Tennessee (soybean), three counties in North Carolina (soybean), 26 counties in Texas (25 soybean) and five counties in Virginia (soybean).

Soybean rust made it deep into the country's soybean growing center, with detections reported for the first time in Iowa. Though the rust was detected in late September, a wet growing season and other favorable climatic factors allowed the crop to mature slightly early and virtually all conventional beans were spared loss from the disease.

“We knew this discovery was a real possibility because of the spore delivery from the south,” said Bill Northey, Iowa secretary of agriculture. “The good news is that since it is so late in the growing season, spraying is not necessary and any infection would not be expected to reduce yields.”

This is the first confirmed case of the disease found during the growing season in Iowa. But Iowa State plant pathologists stressed the discovery coming so late in the growing season is fortunate timing for soybean growers.

In the Southeast, by the time rust moved out of north Florida and south Georgia, virtually all the soybean crop in the Carolinas and Virginia were mature enough to avoid economic damage from the disease.

When rust finally made its way into Virginia, Virginia Tech Soybean Specialist David Holshouser says there was little threat to the 2007 crop.

He reports soybean rust was detected in samples collected from commercial fields in Virginia Beach and Isle of Wight County on Oct. 19. Leaflets were examined on Oct. 24 after 5 days of incubation. Three pustules on two leaflets were found in each sample (48 and 50 leaflets were examined). Rust was not found on samples collected from three prime soybean-producing counties in central and southern Virginia; nor was rust present in three other samples from Isle of Wight County.

“With cooler weather and the recent rains, conditions for rust development are likely to be more favorable. We expect it to spread and develop on any soybeans still having green leaves. However, the soybean crop has matured to a point no yield loss from soybean rust will occur,” Holshouser says.

In Kentucky, rust was detected in early October, but little, if any economic damage was reported, except in extremely late beans. Kentucky Soybean Specialist Don Hershman says, “we continue to look for soybean rust in Kentucky, but have not encountered any new finds beyond those previously reported for Ballard, Caldwell and Fulton counties. Very shortly, there will not be any soybean leaves to look at and we will be forced to look exclusively in kudzu until the first frost hits us.

Hershman says he will make one last scouting run in November, and will monitor movement of rust in Mexico throughout the winter.

In each of the past three years growers have had a good warning system for soybean rust. The grid of grower fields and research plots, referred to as sentinel plots, has provided ample warning in the Carolinas and Virginia. As a result of the sentinel system and a good communication system among soybean specialists and plant pathologists in the area, growers have had timely information that probably paid off more in letting them know when to ‘not’ spray.

In North Carolina, North Carolina State Soybean Specialist Jim Dunphy says, “I think the continued monitoring of sentinel plots to keep track of where rust is and is not is a worthwhile management tool for our soybean producers. In the vast majority of cases, that is where rust has first been found in a county.”

Though fungicides are available to prevent rust and to control it if the disease occurs, soybean specialists have been hesitant to recommend wide-scale of use of preventatives. In addition to costing growers extra money, over-use of these materials has led to rapid development of resistance by a host of disease pathogens.

Dunphy says, “unfortunately, there is little a grower can do management-wise to reduce the risk of getting rust in his soybeans. The earlier maturing varieties have a little less likelihood of getting rust early enough to do economic damage, but also have less time to grow big enough to produce a profitable crop. I'm awfully hesitant to recommend a practice that has a good likelihood of reducing yields and profits whether rust comes or not, to protect against a disease that may or may not come, and which so far has caused very little economic damage in the state.”

Rust did have some impact in South Carolina, causing a number of growers in the eastern part of the state to spray for the disease. Clemson Entomologist John Mueller says, “With forecast for continued dry weather and the advanced growth stage of our soybean crop it looks like we have gotten through the growing season with a minimum of damage due to rust.”

Mueller adds, “quite a few fields were sprayed with fungicides, and I think in most cases this was justified by short intervals of wet weather after spraying. At one rust site researchers recorded an increase of rust from 2 percent of leaves with 1 pustule each to 80 percent of leaves with 50 to 100 pustules each in just two weeks. All the rust in sprayed plots was killed,” Mueller concludes.

Though Asian soybean rust made its way farther west and north in 2007, than in previous years, pathologists are hesitant to make any predictions about 2008. The erratic nature of the movement of the disease makes predictive models very difficult.

The good news for soybean growers in the Southeast is the continued development of soybean sentinel plots that form a picket line from north Florida west to the southern tip of Texas and northward to Canada. If rust occurs, growers have access to this information almost immediately.